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Preparation and characterization of a new magnetic fluid with good low temperature resistance

Nano Research 2025年第1期18卷 477-486页

作者： Shilin Nie Decai Li ✉1 Department of Mechanical Engineering Tsinghua UniversityBeijing 100084China State Key Laboratory of Tribology in advanced equipment Tsinghua UniversityBeijing 100084China

Magnetic fluids are a type of novel functionalmaterial that has emerged over the past two decades, whichhave found applications in various aspects of production anddaily life. However, the application of co.ventional magneticfluids in low-temperature environments is severely limited andoften unsatisfactory. To address this issue, we havedeveloped a new magnetic fluid with excellent low-temperatureresistance. Initially, bare Fe_(3)O_(4) magnetic nanoparticles(FMNPs) were synthesized via co.precipitation without aprotective gas. Subsequently, these particles were modified using polyethylene glyco. (PEG)-4000 as a surfactant. Thebare and modified FMNPs (MFMNPs) were characterized using X-ray diffraction (XRD), vibrating sample magnetometer(VSM), transmission electron microsco.y (TEM), Fourier transform infrared spectrosco.y (FTIR), and thermogravimetricanalysis-differential thermal gravimetric (TGA-DTG) analysis. The rheological properties, low-temperature resistance of themagnetic fluid were also evaluated. The characterization results indicate that the MFMNPs are spherical andmonodisperse, with a narrow size distribution and a mean particle size of approximately 12 nm. Furthermore, the FTIRspectra and TGA-DTA results suggest that PEG-4000 is linked to the bare Fe_(3)O_(4) particles via hydrogen bonding, indicatingsuccessful modification of the Fe_(3)O_(4) magnetic nanoparticles using PEG. VSM measurements demonstrate that surfacemodification does not alter the crystal morphology or superparamagnetic of Fe_(3)O_(4). However, it does reduce the saturationmagnetization from 68.17 to 54.75 emu/g. Additionally, the prepared magnetic fluid exhibits shear thinning and magneticvisco.ity effects. It also exhibits excellent low-temperature resistance, maintaining good fluidity without freezing even at -60 ℃. In summary, these results co.lectively indicate that the new low-temperature resistant magnetic fluid developed in thisstudy is stable and has broad application prospects.

关键词： magnetic fluid low-temperature resistance polyethylene glyco. ethylene glyco. aqueous solution superparamagnetic

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A regulated digital currency

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Science China(Information Sciences) 2019年第3期62卷 119-130页

作者： Yanbing WU Haining FAN Xiaoyun WANG Guangnan ZOU Institute for Advanced Study Tsinghua University Department of co.puter Science and Technology Tsinghua University Space Star Technology co. Ltd

The decentralized digital currency Bitco.n is an anonymous alternative to the centralized banking system and enjoys widespread and increasing adoption. Since Bitco.n created, many other electronic currencies have been developed. We propose a protoco. for an electronic currency for making anonymous payments that can be supervised by an auditor, who has sole access to the transaction history. Other electronic currencies provide only anonymity, which is co.venient for making illegal transactions without regulation. For users, miners, and banks, the transactions of our electronic currency are anonymous, and only auditors can see how it is used. We make use of POW(prove-of-work) technique that allows for distributed decisionmaking within a network, namely the Bitco.n blockchain protoco.. We co.bine the POW and blockchain technology of Bitco.n to give better protection against double-spending attacks.

关键词： Blockchain Bitco.n anonymous digital currency regulated proof-of-work

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Phase-Division-Based Dynamic Optimization of Linkages for Drawing Servo Presses

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Chinese Journal of Mechanical Engineering 2017年第6期30卷 1426-1437页

作者： Zhi-Gang Zhang Li-Ping Wang Yan-Ke Cao JIER Machine-Tool Group co. Ltd Department of Mechanical Engineering Tsinghua University

Existing linkage-optimization methods are designed for mechanical presses; few can be directly used for servo presses, so development of the servo press is limited. Based on the co.plementarity of linkage opti- mization and motion planning, a phase-division-based linkage-optimization model for a drawing servo press is established. co.sidering the motion-planning principles of a drawing servo press, and taking acco.nt of work rating and efficiency, the co.straints of the optimization model are co.structed. Linkage is optimized in two modes： use of either co.stant eccentric speed or co.stant slide speed in the work segments. The performances of optimized link- ages are co.pared with those of a mature linkage SL4- 2000A, which is optimized by a traditional method. The results show that the work rating of a drawing servo press equipped with linkages optimized by this new method improved and the root-mean-square torque of the servo motors is reduced by more than 10%. This research pro- vides a promising method for designing energy-saving drawing servo presses with high work ratings.

关键词： Drawing servo press Linkage optimization Phase-division Motion planning

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OpenMDSP:Extending OpenMP to Program Multi-co.e DSPs

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Journal of co.puter Science & Technology 2014年第2期29卷 316-331页

作者：何江舟陈文光陈光日郑纬民汤志忠叶寒栋 Department of co.puter Science and Technology Tsinghua University Huawei Technologies co. Ltd.

Multi-co.e digital signal processors （DSPs） are widely used in wireless teleco.munication, co.e network transco.ing, industrial co.trol, and audio/video processing technologies, among others. In co.parison with general-purpose multi-processors, multi-co.e DSPs normally have a more co.plex memory hierarchy, such as on-chip co.e-local memory and non-cache-co.erent shared memory. As a result, efficient multi-co.e DSP applications are very difficult to write. The current approach used to program multi-co.e DSPs is based on proprietary vendor software development kits （SDKs）, which only provide low-level, non-portable primitives. While it is acceptable to write co.rse-grained task-level parallel co.e with these SDKs, writing fine-grained data parallel co.e with SDKs is a very tedious and error-prone approach. We believe that it is desirable to possess a high-level and portable parallel programming model for multi-co.e DSPs. In this paper, we propose OpenMDSP, an extension of OpenMP designed for multi-co.e DSPs. The goal of OpenMDSP is to fill the gap between the OpenMP memory model and the memory hierarchy of multi-co.e DSPs. We propose three classes of directives in OpenMDSP, including 1） data placement directives that allow programmers to co.trol the placement of global variables co.veniently, 2） distributed array directives that divide a whole array into sections and promote the sections into co.e-local memory to improve performance, and 3） stream access directives that promote big arrays into co.e-local memory section by section during parallel loop processing while hiding the latency of data movement by the direct memory access （DMA） of a DSP. We implement the co.piler and runtime system for OpenMDSP on PreeScale MSC8156. The benchmarking results show that seven of nine benchmarks achieve a speedup of more than a factor of 5 when using six threads.

关键词： OpenMP multi-co.e digital signal processor data parallelism Long Term Evolution

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Knowledge Map Mining of Financial Data

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tsinghua Science and Technology 2013年第1期18卷 68-76页

作者： Wenhui Shou Wenhui Fan Boyuan Liu Yuyang Lai the Department of Automation Tsinghua University the SOYOTEC Technologies co. Ltd.

The Knowledge Map （KM） co.cept, which was derived to describe and manage knowledge. KM provides insight into the from the Fuzzy co.nitive Map （FCM）, is used interdependencies and uncertainties co.tained in the system. This paper uses a model-free method to mine KMs in historical data to analyze co.ponent stock co.porations of the Shanghai Stock 50 index. The analyses use static and time-domain analyses. The results indicate that a knowledge map is useful for representing knowledge and for monitoring the health of co.panies. Furthermore, sudden changes of the key features of the KMs should be taken seriously by policymakers as an alarm of a crisis.

关键词： Knowledge Map (KM) data mining health monitoring crisis warning

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Fiber optical parametric oscillator based on photonic crystal fiber pumped with all-normal-dispersion mode-locked Yb:fiber laser

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Chinese Physics B 2014年第11期23卷 332-338页

作者：苟斗斗杨四刚张磊王小建陈宏伟陈明华谢世钟陈伟罗文勇 tsinghua National Laboratory for Information Science and Technology (TNList) Department of Electronic EngineeringTsinghua University FiberHome Teleco.munication Technologies co. Ltd

We demonstrate a co.t effective, linearly tunable fiber optical parametric oscillator based on a home-made photonic crystal fiber pumped with a mode-locked ytterbium-doped fiber laser, providing linely tuning ranges from 1018 nm to 1038 nm for the idler wavelength and from 1097 nm to 1117 nm for the signal wavelength by tuning the pump wavelength and the cavity length. In order to obtain the desired fiber with a zero dispersion wavelength around 1060 rim, eight sam- ples of photonic crystal fibers with gradually changed structural parameters are fabricated for the reason that it is difficult to accurately customize the structural dimensions during fabrication. We verify the usability of the fabricated fiber experimen- tally via optical parametric generation and co.clude a successful procedure of design, fabirication, and verification. A seed source of home-made all-normal-dispersion mode-locked ytterbium-doped fiber laser with 38.57 ps pulsewidth around the 1064 nm wavelength is used to pump the fiber optical parametric oscillator. The wide pico.eco.d pulse pump laser enables a larger walk-off tolerance between the pump light and the oscillating light as well as a longer photonic crystal fiber of 20 m superior to the femtoseco.d pulse lasers, resulting in a larger parametric amplification and a lower threshold pump power of 15.8 dBm of the fiber optical parametric oscillator.

关键词： fiber optical parametric oscillator photonic crystal fiber mode-locked fiber laser

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Magnetic Properties of Thermally Aged Fe-Cu Alloys with Pre-deformation

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Journal of Iron and Steel Research International 2016年第9期23卷 981-987页

作者： Yi LI Yuan-fei LI Ben XU Qiu-lin LI Guo-gang SHU Wei LIU School of Material Science and Engineering Tsinghua University Graduate School at Shenzhen Tsinghua University China Nuclear Power Engineering co. Ltd.

Magnetic properties of thermally aged Fe-Cu alloys with pre deformation have been evaluated to improve the understanding of using magnetic technology for the nondestructive evaluation （NDE） of irradiation embrittlement in reactor pressure vessel （RPV） steels. Fe-Cu alloys with and without pre-deformation were thermally aged at 500 ℃ and the changes in microstructure, mechanical properties and magnetic properties were determined. It is found that the strain-induced dislocations reco.er and the Cu-rich particles precipitate during the aging process, and the magnet- ic properties variation depends on the co.bined influence of these two factors. From the point of view of NDE, a fully tempered or annealed microstructure is favorable before RPV is put into service. These results improve the un- derstanding of magnetic property evolution in actual RPV steels and help to develop NDE theory for irradiation embrittlement.

关键词： RPV steel nondestructive evaluation magnetic property Cu-rich precipitate dislocation

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Reaction between Ti–6Al–4V and Y_2O_3–SiO_2 based face shell for investment casting

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Rare Metals 2016年第12期35卷 901-908页

作者： Ya-Meng Wei Zhi-Gang Lu Guang-Peng Wu Xing-Quan Long Department of Mechanical Engineering Tsinghua University State Key Laboratory of Tribology Tsinghua University Guizhou Anji Aviation Investment Casting co. Ltd.

The pH value and visco.ity of Y2O3-SiO2 （Y-Si） slurry made by Y2O3 powders and silica sol for the face co.t of Ti-6Al-4V investment casting were measured. The thermal behavior of the shell made by the Y-Si face co.t system was investigated by differential scanning calorimeter （DSC）, thermal gravimetric （TG） analysis co.bined with mass spectrometry （MS）, and the phase transformations were determined by X-ray diffraction （XRD）. Hot strength, residual strength, linear expansion co.fficient, and wearing resistance performance of the shell were also tested. The microstructure and elements distri- bution of the interaction layer were studied by scanning electron microsco.e （SEM） and energy-dispersive spectrometer （EDS）, respectively. The microhardness tester was applied for the microhardness. The results showed that the slurry was stable for at least 60 h. A very small amount of YZrO3 was formed below 1050℃ and Y2SiO5 was formed around 1450℃. The shell made by Y-Si system had good mechanical property which co.ld reduce cracks during the procedure of dewaxing and inclusions during pouring. Some Al volatilized from the melt, permeated the surface of the face co.t shell, and formed the black reaction layer, which blocked the permeation of O so that O penetration was limited to 5μm. The depth of Si penetration was about 60 μm. The hard layer was also around 60 μm.

关键词： Investment casting Titanium alloy Silica sol Yttrium oxide Interface reaction

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Fatigue damage behavior of a surface-mount electronic package under different cyclic applied loads

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Chinese Physics B 2014年第4期23卷 377-384页

作者：任淮辉王习术 Longyuan Beijing) Wind Power Engineer Technology co. Ltd Department of Engineering Mechanics Tsinghua University

This paper studies and co.pares the effects of pull-pull and 3-point bending cyclic loadings on the mechanical fa- tigue damage behaviors of a solder joint in a surface-mount electronic package. The co.parisons are based on experimental investigations using scanning electron microsco.y （SEM） in-situ technology and nonlinear finite element modeling, respec- tively. The co.pared results indicate that there are different threshold levels of plastic strain for the initial damage of solder joints under two cyclic applied loads; meanwhile, fatigue crack initiation occurs at different locations, and the accumulation of equivalent plastic strain determines the trend and direction of fatigue crack propagation. In addition, simulation results of the fatigue damage process of solder joints co.sidering a co.stitutive model of damage initiation criteria for ductile materials and damage evolution based on accumulating inelastic hysteresis energy are identical to the experimental results. The actual fatigue life of the solder joint is almost the same and demonstrates that the FE modeling used in this study can provide an accurate prediction of solder joint fatigue failure.

关键词： solder joint fatigue life hysteresis energy SEM in-situ technology

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Hurricane damage assessment for residential co.struction co.sidering the non-stationarity in hurricane intensity and frequency

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Acta Oceanologica Sinica 2016年第12期35卷 110-118页

作者： WANG Cao LI Quanwang PANG Long ZOU Aming ZHANG Long Department of Civil Engineering Tsinghua University CCCC Highway co.sultants co. Ltd.

Natural hazards such as hurricanes may cause extensive eco.omic losses and social disruption for civil structures and infrastructures in co.stal areas, implying the importance of understanding the co.struction performance subjected to hurricanes and assessing the hurricane damages properly. The intensity and frequency of hurricanes have been reported to change with time due to the potential impact of climate *** this paper, a probability-based model of hurricane damage assessment for co.stal co.structions is proposed taking into acco.nt the non-stationarity in hurricane intensity and frequency. The nonhomogeneous Poisson process is employed to model the non-stationarity in hurricane occurrence while the non-stationarity in hurricane intensity is reflected by the time-variant statistical parameters(e.g., mean value and/or standard deviation), with which the mean value and variation of the cumulative hurricane damage are evaluated explicitly. The Miami-Dade co.nty, Florida, USA, is chosen to illustrate the hurricane damage assessment method proposed in this paper. The role of non-stationarity in hurricane intensity and occurrence rate due to climate change in hurricane damage is investigated using some representative changing patterns of hurricane parameters.

关键词： hurricane damage assessment intensity frequency non-stationarity climate change

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